Anal. Chem. 1984, 56,985-988 ( I O ) Levich, V. G. "Physicochemical Hydrodynamics", 2nd ed.; PrenticeHall: Englewood Cliffs, NJ, 1962; p 20. (11) Matsuda, H. J. Nectroanal. Chem. 1967, 15, 109. (12) Yamada, J.; Matsuda, H. J . Etectroanal. Chem. 1973, 44, 189. (13) Weber, S.G. J. Neckoanal. Chem. 1983, 145, 1. (14) Gunasingham, H.; Fleet, B. Anal. Chem. 1983, 55, 1409. (15) Matsuda, H. J. Electroanal. Chem. 1968, 16, 153. (16) Shoup, R. E., personal communication. (17) Meschi, P. L.; Johnson, D. C. Anal. Chem. 1980, 52, 1304. (18) Littlewood, A. B. "Gas Chromatography: Principles, Techniques, and Applications", 2nd ed.; Academic Press: New York, 1970. Wightman, R. M. Anal. Chem. 1982, 54, (19) Caudill, W. L.; Howell, J. 0.; 2532.
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(20) Weisshaar, D. E.; Tallman, D. E.; Anderson, J. L. Anal. Chem. 1981, 53, 1809-1813.
RECEIVED for review November 23,1983. Accepted February 6,1984. Acknowledgment is made to the donors of the Petroleum Fund, administered by the American Chemical Society, for partial support of this research and to the National Of under Grant GM28112, for Institutes Of this research.
Simultaneous Conductivity and Permittivity Detector with a Single Cell for Liquid Chromatography J. F. Alder* and P. R. Fielden DIAS, The University of Manchester Institute of Science a n d Technology, P.O. Box 88, Manchester M60 IQD, United Kingdom
A. J. Clark Chemical Defence Establishment, Porton Down, Salisbury, Wiltshire SP4 OJQ, United Kingdom
A conductlvlty detector Is descrlbed which Is united with a permlttivlty detector to provide simultaneous measurement wlthin a single cell. The combined system Is used to detect eluted species from reversed-phase HPLC where the conductivity monitor Is selectlve toward lonlzed/partlally Ionized molecules. The conductlvlty detector performance Is demonstrated by the quantlflcatlon of an organophosphorus acid under reversed-phase conditions and by the detectlorl of separated anions for nonsuppressed ion chromatography, where a detection limit of 50 ppb CI- In a 100 pL sample volume Is presented.
Permittivity and conductivity are bulk properties which have lent themselves well to detection of solutes eluted from high-performance liquid chromatography columns. Previous work (1-3) has shown the versatility of permittivity detection although it is recognized that for many applications the technique cannot demonstrate adequate sensitivity and it does not perform well if the eluent is electrically conductive. The similarity between cell requirements for permittivity and conductivity measurements has led workers t o combine the two approaches. A detector that monitored either permittivity or conductivity by using two RC monostable circuits connected in series to give a variable mark-space ratio astable circuit was reported by Hashimoto e t al. (4). A cell, similar t o that of Vespalec (5) was used to detect alkaloids and aliphatic amines in normal phase eluents and carboxylic acids (C8-C22)in reversed-phase eluents. Johansson e t al. (6) demonstrated the use of a high-frequency capacitance cell for the continuous monitoring of taurine and glycine conjugated bile acids from reversed-phase partition chromatography. Jackson (7) clamped two aluminum electrodes, around the end of a Sephadex glass column, with 1 mm separation. A 1-10 MHz radio frequency (rf) coupling technique was used and a response to 1 X g of NaCl claimed, although the
noise level was apparently ten times this response. Pecsok and Saunders (8) mounted a cell at the base of a column and used two platinum-rhodium mesh disks, as electrodes, to give an effective volume of -5 pL. A commercial conductivity meter was employed for the measurement. A cell, with parallel plate platinum electrodes, suitable for HPLC was presented by Tesdik and Kalftb (9) with a volume of C0.5 pL. Detection, using a commercial conductivity bridge, gave a sensitivity of 1 X g mL-l of KC1 for peaks eluted from a column filled with glass beads. An attenutation detector with logarithmic response over 6 decades of conductivity, excited with a 2.5 kHz square wave was reported by Svoboda and Marsal(10). They used a 3 pL capacity cell with stainless steel electrodes and detected peaks with steep band edges which would have required range switching on conventional conductivity instruments. Alder and Drew (11) described a conductivity bridge device, operated at 115 MHz with a straight glass tube acting as a cell around which an inductor was wound. A cell volume of -9 p L was claimed with a sensitivity of 2 ppm KC1 in water. The monitoring of conductivity changes, in addition to permittivity changes, is attractive since these measurements complement each other. Permittivity detection is particularly ubeful with nonpolar eluents while conductivity detection is applicable in conducting polar eluents where it performs as a bulk property monitor. The conductivity detector is particularly sensitive toward ionic species in an eluent of low conductivity. Ion chromatography (12,13)developed by Small and co-workers (14) is a major application of conductivity monitors. The cell requirements for both detectors are very similar; both employing a two-electrode cell. Measuring both permittivity and conductivity with the same cell is particularly attractive, since it eliminates the need to use two cells connected in series which would lead t o increased band broadening within the second cell and a time lag between signals. This paper describes the design of a combined conductivity/permittivity monitor based on the device described in our previous work (3). A single cell is coupled to both detectors
0003-2700/84/0356-0985$01.50/0 0 1984 American Chemical Society
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Figure 1. Conductivity detector measurement circuit.
t o provide simultaneous measurement. T h e scope of the combined system is illustrated with reversed-phase separations. T h e performance of the conductivity detector is assessed through the separation of anion mixtures by nonsuppressed ion chromatography and the reversed-phase separation of organophosphorus acids.
Flgure 2. A block diagram of the conductivity detector showing its coupling to the permittivity detector and measurement cell: (1) permittlvity detector: (2) osclllator; (3) noninverting amplifier; (4) voltage subtractor: 15) rectifier; (6) integrator; (7) offset ampllfler; (8) low pass filter. .
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Design of the Permittivity/Conductivity Detector. The suitability of the conical plate cell (2,3) (shown in ref 3, Figure 3) for conductance measurement was investigated from a consideration of cell geometry. It was shown that the resistance of such a cell (Rcu) having cone base diameter (d) and axial height (1) filled with medium of conductance (a) is RcELL
= 21/ad2a
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The circuit designed to monitor conductivity changes within the conical plate cell was a low frequency contacting system measuring changes in attenuation. The principle used was similar to that of Svoboda and Marsal (IO) but employed different techniques of signal recovery. Figure 1 shows the heart of the measurement circuit: a noninverting amplifier fed with an 7.4-kHz ac wave form of amplitude V, symmetrical about zero. The amplitude of the output wave form is
R1 is a fixed resistor, while Z is the impedance of a series RC circuit, of which the cell forms the resistive component RcELL. It can be shown that, if R2CELL > X Z c and R1 = 1 kQ then
for input voltage Vm = 1V. The input wave form is symmetrical about zero to minimize possible problems due to cell polarization; no evidence for such problems was observed in practice. Equation 3 predicts a linear change in output voltage with conductance. Sensitivity can be increased by reducing the cell plate spacing, which simultaneously increases the sensitivity to permittivity change ( I ) . Figure 2 shows a block diagram of the conductivity detector circuit. The 1V p-p square wave was fed to the noninverting input of the measurement amplifier. The difference between the signal amplitude of the oscillator and the measurement amplifier output was obtained by use of a voltage subtractor. The resulting difference voltage was rectified and integrated to give a dc voltage proportional to the cell conductance. A further amplifier was used to control the signal offset. A fiial amplifier was used as a low-pass filter and buffer with a variable filter constant of up to 0.4 s. The voltage output was reproduced with a chart recorder, which also acted as a scaling control for the final trace of conductance. It was necessary to incorporate the detector cell into both the 25-MHz permittivity cell oscillator and conductivity circuits without either circuit influencing the other. Figure 2 shows the coupling arrangement employed. The HF signal was filtered out of the conductivity detection circuit by the rf choke which offered only a negligible contribution to the measured conductance. Low-frequency signals from the conductivity circuit were isolated from the permittivity oscillator by a 33 pF series cell capacitance C,. This arrangement allowed
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Figure 3. Determination of ethylmethylphosphonic acid (EMP) in 60:40 methanoVwater on an ODS column: flow rate, 2 mL min-', 10 pL sample; (A) 10 ng EMP, (B) 10 pg EMP; (1) EMP, (2) water.
successful simultaneous monitoring of both permittivity and conductivity with no mutual interference. ChromatograRhic Measurements. The HPLC system comprised an LC pump, Model 410 (Kontron Analytical), a pneumatic pulse damper, and valve injector (Rheodyne 7125). Investigation of the conductivity detector operation was performed with a reversed-phase system (6040 methanol/water with a HYP ODs-2509 column) for separation of organophosphorus acids. Ethylmethylphosphonic acid (EMP) was injected onto the ODS column in lO-bL samples of a series of dilutions made up in 6040 methanol/water mixture. A flow rate of 2 mL min-' was used for 60:40 methanol/water eluent (the water was singly distilled without further purification). The acidic nature of EMP meant that it passed through the column unretained and emerged as a single peak with a solvent peak due to water. Figure 3 shows separate injection of samples containing 10 gg and 10 ng of EMP. The calibration curve for 10 ng-10 bg EMP showed slightly negative curvature on a log-log axis, with the voltage output measured changing from 2 mV at 10 ng to 10 V at 10 pg of EMP injected.
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Detection of separated anions by conductivity measurement: column, anion exchange (Vydac 302); mobile phase, 5 X M potassium hydrogen phthalate (pH 4.2),2 mL min-'; 100 pL sample containing 16.7 ppm of each anion; (1) CI-, (2) NO2-, (3)Br-, (4)NO3-, (5)SO4*-, (6) I-. Flgure 6.
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Simultaneous monitoring of a mixture of acetone, nitrobenzene, and phenol separated on an ODs column by conductivity and permittivity measurement: moblle phase, 60:40 methanol/water, 2 mL mln-'; 10 pL sample; (1) solvent peak, (2) acetone, (3) nitrobenzene, (4) phenol. Flgure 4.
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Determination of anions in tap water with conductivity detection: column, anion exchange (Vydac 302); mobile phase, 5 X lo3 M potassium hydrogen phthalate (pH 4.2),2 mL min-'; 100 pL sample; (1)CI-, (2)NO3-, (3)S O:-, (4)HC03-. Flgure 7.
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Simultaneous detection of fructose by conductivity and permittivity measurement: column, NH2 (7 pm); mobile phase, 80:20 acetonitriidwater, 1 mL min-'; 20 pL sampie of 10% fructose in water; (1) solvent peak, (2)fructose. Flgure 5.
Similar results were obtained for methylphosphonic acid. A mixture of acetone, nitrobenzene, and phenol was dissolved in 6040 methanol/water eluent and injected as a 10-pL sample onto the ODS column. A flow rate of 2 mL min-l was maintained and a separation effected. Figure 4 shows a typical result: the permittivity detector responded to all three components, while the conductivity monitor showed a weak response to phenol only (phenol K , = 1.3 X 10-lo). The chromatography of the sugars fmctose, glucose, and sucrose was investigated. These compounds are difficult to detect by UV methods and previous investigations had shown that they were detectable with a permittivity detector (1).
An "-NH2" column (Zorbax, NH2) was used with 8020 acetonitrile/water eluent at a flow rate of 1mL min-'. Samples of the three sugars in water were injected as 20-pL aliquots. Glucose and sucrose were retained on the column for longer than fructose and resulted in poor chromatographic peaks which were broad and difficult to detect above base line noise. The high back pressure of the column prevented the use of faster eluent flow rates. The response of the permittivity and conductivity detectors to fructose (- 10% solution in water) is shown in Figure 5. Both detectors responded to the fructose as well as the solvent peak albeit with little sensitivity,but the conductivity monitor detected an additional negative peak. Separation of anions by nonsuppressed ion chromatography was performed by using an anion exchange column (Vydac 302) with 5 X M potassium hydrogen phthalate (KHP) eluent adjusted to pH 4.2 with 0.1 M NaOH solution. A sample loop of 100 pL volume was used throughout. KHP eluent is suited to conductivity detection due to its low background conductance (15).
A chromatogram of a mixture of six anion standards is shown in Figure 6. The anion analysis of tap water is illustrated in Figure
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t/min Figure 8. Separation of CI- near the limit of detection: sample, 100 I!of 50 ppb CI-; (1) water dip, (2)CI- (50 ppb).
7, showing an important application of ion chromatography/ conductivity detection. A calibration for C1- was generated over the range 50 ppb to 100 ppm. Figure 8 shows a chromatogram near the detection limit. The calibration curve is linear up to 50 ppm C1- with slight negative curvature at high concentrations which is mainly an artifact of the chromatography. The detection limit of
